A plate making method of exposing a positive working photosensitive lithographic plate, developing it, and subjecting it to heat processing (hereinafter referred to as burning-in processing) is known as disclosed in British Patents 1,151,199 and 1,154,749. Burning-in processing is usually carried out at from 200.degree. to 320.degree. C. for 1 to 60 minutes. This processing hardens the image area to provide markedly improved impression capacity but, at the same time, causes an ink-receptive organic substance dissolved out of the dot image area to firmly adhere to the non-image area, especially shadows, which appears as stains on printing. This phenomenon is called burning-in ink spreading. It is known that burning-in ink spreading becomes conspicuous as the temperature of burning-in increases.
In order to overcome this problem, JP-B-1-49932 (the term "JP-B" as used herein means an "examined Japanese patent publication") mentions hydrophilic additives for increasing impression capacity after burning-in processing. It is described that impression capacity equal to that reached by burning-in processing at a generally employed burning-in processing temperature of from 200.degree. to 320.degree. C. can be attained by a processing at a reduced burning-in processing, e.g., about 180.degree. C. In fact, however, even processing at 180.degree. C. still causes burning-in ink spreading. It also turned out that the burning-in ink spreading considerably occurs if burning-in processing is effected at a generally employed temperature. Therefore, the above technique offers no basic solution to the problem. Additionally, the hydrophilic additives should be used in a large quantity, which results in serious reductions in other printing properties, such as development latitude and ink-receptivity.
Having high hydrophilic properties, these additives have insufficient solubility in solvents preferably used for coating, such as methyl ethyl ketone and, if added in large quantities, tend to dissolve out from the image area during development.